RIBONUCLEIC ACID (RNA)

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An understanding of the nucleic acid molecules which include deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) is important for us to understand the molecular and/or genetic basis of life. DNA is the genetic material of the cell. Nucleic acid molecules are found in all living organisms as the carriers of genetic materials from parent organisms to their offspring’s. With the exception of viruses (which contain either a DNA or an RNA genome, and not both in one organism) all other living organisms contain both DNA and RNA as their nucleic acid molecules. These nucleic acid molecules are part of the essential macromolecules of life.

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Other macromolecules of life include proteins, carbohydrates and lipids. Ribonucleic acid (RNA) is a polymeric nucleic acid molecule that is made up of one or more nucleotides that contain the pentose sugar ribose. RNA is one of the two types of nucleic acid molecules found in all living cells. The major biological function of the RNA in the cell is in the transmission of genetic information from DNA; and in the end the genetic information encoded in the DNA is translated into specific protein molecules. The synthesized protein molecules are the major instruments through which the genetic potential of an organism (as encoded in the DNA) are expressed in the cell. RNA unlike DNA (which is double stranded in nature) is single stranded (Figure 1).

Figure 1. Illustration of the double helix structure of deoxyribonucleic acid (DNA) and single stranded structure of ribonucleic acid (RNA).

DNA and RNA are in the form of strands of molecules that resembles a ladder. A strand of DNA is composed of four nitrogenous bases known as adenine, guanine, thymine, and cytosine. RNA is different from DNA because it does not have thymine. RNA has uracil as a base in place of thymine (which is only present in the DNA molecule). DNA and RNA are critical to all living things. DNA contains the genetic information that makes every living thing what they are while RNA acts as a tool that enables cells to make the proper and specific protein molecules (as encoded in the DNA) that carryout the other function of the cell. The overall structure of RNA and DNA consists of four nitrogenous bases that compose the backbone of their strands.

RNA as a nucleic acid molecule performs several biological functions in every living organism. Most notably is the fact that RNA molecules helps in the expression of genes in a living organism. Their other functions are in the coding, decoding, and regulation of gene expression in the cell of an organism including microorganisms. RNA molecules carry out the genetic information encoded in the DNA especially when protein synthesis is involved. RNA molecules can also act as enzymes in living organisms.

Ribozymes are RNA molecules that function as catalysts and/or enzymes in living organisms, and they are found in both eukaryotic and prokaryotic cells – where they help to process RNA precursor molecules especially during protein synthesis. Ribozymes also known as catalytic RNA are found in the ribosomes where they help to join amino acids together to form protein chains. According to the central dogma of molecular biology, the flow of genetic information in a living cell is from the DNA through the RNA and then to proteins. In order words, the DNA makes RNA while the RNA makes the protein molecules. All the genetic materials required for the reproduction of new cells, tissues and/or the whole organism is contained in the DNA of the cell.

When the cell of an organism needs to synthesize a particular protein molecule, the protein’s gene (i.e., the portion of the DNA that codes for that particular protein to be synthesized) becomes activated. And once this is done, multiple copies of that piece of DNA in the form of messenger RNA (mRNA) will be immediately synthesized. These copies of mRNA so synthesized are then used to translate the genetic code (contained in the DNA molecule) into specific protein molecules through the action of the cell’s protein manufacturing machinery. The protein manufacturing machinery of the cell is known as the ribosomes. Ribosomes are the sites for protein synthesis in the cell of an organism.  

Protein molecules are usually made up of polypeptides, which are chains of amino acids linked together by peptide bonds. Protein synthesis in the cell is a multifaceted biological process in which the main constituents of the protein molecule (inclusive of the amino acids and protein structure) expressed in the cell is encoded and directed by the genetic information encoded in the DNA. Proteins expressed in the cell are usually encoded by the genetic information in the DNA, but this varies for viruses whose nucleic acids are either DNA or RNA.

Thus, proteins can also be encoded in viral particles by the RNA. For protein synthesis to start, an RNA copy of the specific DNA molecule that encodes the particular protein to be biosynthesized must be synthesized first through the process of transcription. And it is this RNA copy that directs the synthesis of the protein in the cell (inclusive of eukaryotic and prokaryotic cells) through the process of translation. There are three major types of RNA molecules that participate in the protein-synthesizing pathway via which proteins are synthesized in the cell of an organism.

The three types of RNA molecule are highlighted in this section.

  1. Messenger RNA (mRNA)
  2. Transfer RNA (tRNA)
  3. Ribosomal RNA (rRNA)    
  • Messenger RNA (mRNA): Messenger RNA (mRNA) carries the genetic information copied from the DNA in the form of a series of three-base code words known as codons; and each of these codons specifies a particular amino acid. Amino acids are the main building blocks of proteins. Amino acids are the monomers required for the synthesis of proteins, which are macromolecules. mRNA is the RNA that carries the actual genetic information for the synthesis of particular proteins from the DNA molecule.
  • Transfer RNA (tRNA): Transfer (tRNA) carries amino acids to the ribosome of the cell where they are assembled into various protein molecules. They are the key to deciphering the code words in the mRNA molecules.
  • Ribosomal RNA (rRNA): Ribosomal RNA (rRNA) is found in the ribosomes. They associate with certain proteins to form the ribosomes. rRNA basically serves as the structural components of protein-making structures known as ribosomes.

FURTHER READING

Alberts B, Bray D, Lewis J, Raff M, Roberts K and Watson J.D (2002). The molecular Biology of the Cell. Fourth edition. New York, Garland, USA.

Cooper G.M and Hausman R.E (2004). The cell: A Molecular Approach. Third edition. ASM Press.

Dale J (2003). Molecular genetics of bacteria. Jeremy W. Dale and Simon Park (4th eds.). John Wiley & Sons Ltd, West Sussex, UK. Pp. 312-313.

Lewis R (2004). Human Genetics: Concepts and Applications. Sixth edition. McGraw Hill Publishers, USA.

Robert L. Nussbaum, Roderick R. McInnes and Huntington F. Willard (2001). Genetics in Medicine. Philadelphia, USA. Saunders publishers.

Sambrook, J., Russell, D.W. (2001). Molecular Cloning: a Laboratory Manual, 3rd edn. Cold Spring Harbor Laboratory Press, New York.

Tamarin Robert H (2002). Principles of Genetics. Seventh edition. Tata McGraw-Hill Publishing Co Ltd, Delhi.     

Twyman R.M (1998). Advanced Molecular Biology: A Concise Reference. Bios Scientific Publishers. Oxford, UK.

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